1. Field of the Invention
The present invention relates to a system of electronic reprographics and, more particularly, to a system of electronic reprographics in which scanning motion and image quality are analyzed and corrected as necessary.
2. Description of the Related Art
In light lens printing systems, a lamp or flashing unit flashes light on a document and has an image created synchronously on a photoreceptor belt. The photoreceptor belt picks up toner from which a copy is made.
In electronic reprographic printing systems, a document or series of documents comprising at least one print job are successively scanned. Upon scanning of the documents, image signals are obtained and electronically stored. The signals are then read out successively and transferred to a printer for formation of the images on paper. Once a document is scanned, it can be printed any number of times or processed in any number of ways (e.g., words deleted or added, image magnified or reduced, etc.). If a plurality of documents comprises a job which is scanned, the processing or manipulation of the scanned documents can include deletion of one or more documents, reordering of the documents into a desired order, or addition of a previously or subsequently scanned document or documents. The printing or processing can be relatively synchronous with scanning, or asynchronous after scanning. If asynchronous, a time interval exists between scanning and printing or processing. The system can then accumulate a number of scanned jobs in the system memory for subsequent processing or printing. The order of the jobs to be printed may be different from the order of jobs as scanned depending on the priority of the jobs and the desires of the operator for increasing productivity or through-put and decreasing printer or scanner down-time.
Scanning of documents can be accomplished by providing a scanning mechanism for focusing selected portions of the document upon photosensitive cells which generate electrical signals related to the light level falling upon the cells. One problem with document scanners is the difficulty of adjusting the position of the photosensitive cells to obtain the desired location of the image upon the photosensitive cells. This problem is particularly prevalent in electronic reprographic printing systems which require a high degree of accuracy of the position on each scanning line.
A number of methods and apparatus are known for scanning documents. For example, U.S. Pat. No. 4,464,681 to Jacobs et al discloses a method and apparatus for adjusting a facsimile document scanner in which a test pattern is used to adjust the facsimile. In particular, a test pattern is printed on a piece of paper which is adhered to the inside of the facsimile for calibrating the facsimile. When the image of the test pattern is the same size as the photodiode array and when the image is precisely focused upon the photodiode array, the dark and light image stripes of the pattern fall exactly upon alternate photocells of the photodiode array.
U.S. Pat. No. 4,605,970 to Hawkins discloses a diagnostic tool for calibrating an optical document digitizer, comprising a strip composed of an optical pattern of contrasting light and dark areas on one surface thereof. Such strips or ladder charts provide a crude method of determining the position of the document being scanned. A number of scanning lines must be read in order to determine location.
U.S. Pat. No. 4,749,872 to Asada et al provides a target for reference of home positions in a document scan apparatus. A reference position marker is mounted on the document support table which directly reflects light from a document illumination light source into an optical sensor array through a mirror and a lens which causes the array to operate in a saturated range to generate a higher output signal level than an expected highest signal level of the sensor array.
U.S. Pat. No. 4,641,357 to Satoh discloses an apparatus for scanning a document image wherein a single reference strip is provided along the side of a moving platen. The reference strip comprises a pattern of optically scannable reference lines inclined at a 45.degree. angle with respect to the CCD and the platen. An analyze circuit is provided for determining whether a right edge of a black stripe of the lines is at a detect position in which case image data of the respective line is gated to a buffer memory of an image process circuit and the detect position is shifted to the next picture element position. If the condition is not satisfied, the image data is discarded (not supplied to the buffer memory) and the detect position is unchanged. Using the stripe pattern of lines, the system is effectively rendered insensitive to interruptions of document motion and/or variations in document motion speed.
With electronic reprographic printing systems, the position of the document must be precisely controlled. Methods and apparatus are therefore required which can determine any displacements in a scan line position and to correct these displacements. It is further desirable to diagnose and correct motion and vibratory problems within such a system.